Field of the Invention
The present invention relates to a process for forming composite lignocellulosic materials such as wood products. More particularly, the present invention relates to a process that bonds at least two lignocellulosic substrates together to form a composite without an added adhesive.
Description of Related Art
The forest products industry has a unique opportunity to influence future building materials based on its low energy requirements for manufacturing and high performance capabilities. Engineered wood products take advantage of wood's natural strengths in certain directions for specific purposes. However products like plywood, oriented strand board, and laminated veneer lumber require the use of petroleum derived adhesives for bonding. Formaldehyde based resins and polymeric isocyanates are common because of their availability and cost benefits. Phenol-formaldehyde (PF) and Urea-Formaldehyde (UF) are synthesized from two non-renewable resources of natural gas and petroleum. Further, PF and UF adhesives are known to emit gases during production and product life that are classified as carcinogenic and can be nauseating upon significant exposure. See An Introduction to Indoor Air Quality, in: Agency E.P. (Ed.), Washington D.C., 2010, p. 4. Since 2000, governmental restrictions on wood composite adhesives has led researchers to look for wood adhesives that are sustainable, emit no harmful gases, economical, and yield performance on par with current formaldehyde wood composites. See Pizzi A., Recent developments in eco-efficient bio-based adhesives for wood bonding: opportunities and issues, Journal of Adhesion Science Technology, 20 (2006) 829-46 (“Pizzi, 2006”). Although there is research in a wide variety of renewable adhesives including tannins, carbohydrates, soy protein, and wood welding, there is only a single commercial alternative to phenol formaldehyde (PF) adhesives which is soy-based. See Pizzi, 2006. While significant progress has occurred in addressing critical issues of renewable and sustainable adhesives, the soy-based adhesive still requires some petroleum-based additives. See Pizzi, 2006. Thus, there is a clear need for alternative production methods in the wood product manufacturing industry.
With thermoplastic and metallic substrates, adhesion for the same materials can easily occur via heat as the liquid-like surfaces achieve intimate contact and form a ridged joint upon cooling. A developing area of work is in the area of laser welding that is applied industrially. Laser technology is used in many industrial materials applications for precision machining in addition to joint welding. Previous research on the laser modification of lignocellulosic materials has focused on automated machining and cutting, with other studies demonstrating utility of lasers for cleaning historical documents and wood surfaces, pyrolysis, accelerated weathering, and microtoming. See McMillin C. W., Laser machining of southern pine, Forest Products Journal, 21 (1971) 34-7 (“McMillin C. W., 1971”); C. C. Peters, H. L. Marshall, Cutting wood materials by laser, U.S. Forest Products Laboratory, Madison, Wis., 1975 (“Peters et al., 1975”); Seltman I., Freilegen der Holzstruktur durch Uv-Bestrahlung, European Journal of Wood and Wood Products, 53 (1995) 225-8; Kolar I, Strlic M., Muller-Hess D., Gruber A., Troschke K., Pentzien S., et al., Near-UV and visible pulsed laser interaction with paper, Journal of Cultural Heritage, 1 (2000) S221-S4 (“Kolar et al., 2000”); Hendrik Wust et al., Laserinduzierte Modifikationen an Holzoberflachen, Wissenschaftlich Zeitschrift der Technischen Universitat Dresden, 48 (1999) 66-72. As the polymeric components of wood are sensitive to high energy levels it is logical that these studies have focused in the above applications.
However, the earliest studies on the interaction of laser light and wood indicated that laser irradiation causes the wood surface to undergo flow. See Nordin S. B., Nyren J. O., Back E. L., An Indication of Molten Cellulose Produced in a Laser Beam, Textile Research Journal, 44 (1974) 152-4; Wust H., Haller P., Wiedemann G., Experimental study of the effect of a laser beam on the morphology of wood surfaces, 2007 (“Wust et al., 2007”); Parameswaran N., Feinstrukturelle Veriinderungen an durch laserstrahl getrennten Schnittflachen von Holz and Holzwerkstoffen, European Journal of Wood and Wood Products, 40 (1982) 421-8. The laser modified surfaces appear to have a glossy reflective surface as seen in light microscopy images. At higher magnification using a scanning electron microscope, cells were no longer clearly evident and appear to have undergone flow. Interestingly, the studies suggest that the wood surface reach a liquid-like, rubbery state during the process of laser irradiation, dependent upon the laser energy.
The novel observation that wood can be auto-adhered using frictional heat was demonstrate in a number of studies that explored how adhesive joints could be formed without added adhesive. Pizzi and co-workers applied pressure arid oscillatory vibration creating heat from friction to bond wood substrates. See Balz Gfeller et al., Solid Wood Joints by In Situ Welding of structural wood constituents, Holzforschung, 58 (2004) 45-52; Bocquet J., A Pizzi, L Resch, Full-scale (industrial) wood floor using welded-through dowels, Journal of Adhesion Science Technology, 20 (2006) 1727-39; P Omrani, H R Mansouri, G Duchanois, A Pizzi, Fracture mechanics of linearly welded woodjoints: effect of wood species and grain orientation, Journal of Adhesion Science Technology, 23 (2009) 2057-72; C. Ganne-Chedeville, M. Properzi, I M Leban; A Pizzi, F Picheilin, Wood Welding: Chemical and Physical Changes According to the Welding Time, Journal of Adhesion Science Technology, 22 (2008) 761-73. These studies identified lignin and hemicelluloses underwent softening and flow while cellulose fibers entangle, creating an entanglement network in a matrix of lignin and hemicellulose.
Significant variables identified in previous studies understanding laser modification of wood include laser wavelength (see Kolar et al., 2000; Wust et al., 2007), laser fluence (see C. C. Peters, H. L. Marshall, Cutting wood materials by laser, U.S. Forest Products Laboratory, Madison, Wis., 1975), laser movement to wood grain orientation (see McMillin C. W., 1971; Lee C. K., Chaiken R. F., Singer J. M., Charring pyrolysis of wood in fires by laser simulation, Symposium (International) on Combustion, 16 (1977) 1459-70; Barcikowski S., Koch G., Odermatt J., Characterization and modification of the heat affected zone during laser material processing of wood and wood composites, European Journal of Wood and Wood Products, 64 (2006) 94-103 (“Barcikowski et al., 2006”)), moisture content (see Barcikowski et al., 2006; Wust H., Beyer E., Morgenthal L., Panzner M., Wiedemann G., Fischer R., et al., Laserinduzierte Modifikationen an Holzoberflachen, (1999)) species (see Vladimir Necesany M. K., Chemical Substance of the Glassy Amorphous Material Effused from Beech Wood During the Cutting by Laser Radiation, Bratislava Statny, 110 (1986) 13-27), and specific gravity (see McMillin C. W., 1971; Peters et al, 1975). Even further, a laser-assisted joining device is described in U.S. Published Patent Application No. 20030159294, which provides an apparatus and method for joining together the surfaces of wood by introducing laser light between the surfaces just before the surfaces are forced together and a pressure roller directly applies force at the line of initial contact between the two surfaces to ensure close contact between the closing surfaces at the same time that the closing surfaces are illuminated by the laser light. Even in light of such developments, there still exists a need for wood composites bonded in a manner that provides a strong union without adhesives.